Allow transcription-triggered interruptions (fixes double-reply collision)

Live call: caller said "this is Carlos", the reply started, caller continued
"How are you doing today?" - with all interruptions off, both replies queued
and played back-to-back (~10s of bot monologue answering two different
things). Middle ground: VAD-triggered interruptions stay OFF (raw VAD fires
on noise with no transcript - that was the dead-air bug), but transcription-
triggered interruptions are back ON: only real transcribed caller speech can
cancel a stale queued reply. Noise cannot produce a transcript, so the
dead-air regression is structurally impossible.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

bot.py

@@ -730,11 +730,18 @@ async def run_agent(transport, caller_number=None, call_sid=None, do_capture=Tru
             )
         )
     ]
+    # VAD start: interruptions OFF — raw VAD fires on breaths/noise with no transcript and
+    # used to silently cancel queued replies (20-35s dead air). Transcription start:
+    # interruptions ON — only REAL transcribed caller speech may cancel a stale queued
+    # reply. Noise can't produce a transcript, so the dead-air bug can't return; but when
+    # the caller keeps talking as a reply starts (live call 2026-07-04: "this is Carlos" /
+    # reply starts / "How are you doing today?"), the outdated reply is cancelled instead
+    # of both replies playing back-to-back.
     user_params = LLMUserAggregatorParams(
         user_turn_strategies=UserTurnStrategies(
             start=[
                 VADUserTurnStartStrategy(enable_interruptions=False),
-                TranscriptionUserTurnStartStrategy(enable_interruptions=False),
+                TranscriptionUserTurnStartStrategy(enable_interruptions=True),
             ] if HALF_DUPLEX else None,  # None -> library defaults (interruptions on)
             stop=stop_strategies,
         ),